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POC: multi-threaded dispatch on wasm32-unknown-wasip1-threads #6

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POC: multi-threaded dispatch on wasm32-unknown-wasip1-threads #6

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269c8a2
docs(wasi-threads): record toolchain, sysroot, and runtime findings
scottmarchant Aug 18, 2026
ef888d4
build(wasi-threads): add a DISPATCH_WASI_THREADS toolchain knob
scottmarchant Aug 18, 2026
b593221
feat(wasi-threads): compile libdispatch for wasm32-unknown-wasip1-thr…
scottmarchant Aug 18, 2026
65fb871
test(wasi-threads): prove dispatch_async runs on worker threads
scottmarchant Aug 18, 2026
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8 changes: 7 additions & 1 deletion CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -207,7 +207,13 @@ if(CMAKE_SYSTEM_NAME STREQUAL "WASI")
set(HAVE_STRLCPY 1 CACHE BOOL "WASI libc result")
set(HAVE_SYSCONF 1 CACHE BOOL "WASI libc result")
set(HAVE_ARC4RANDOM 1 CACHE BOOL "WASI libc result")
set(USE_POSIX_SEM 0 CACHE BOOL "WASI libc result")
if(DISPATCH_WASI_THREADS)
# wasip1-threads: sem_t is a real blocking semaphore (proven under
# wasmtime v24, including sem_timedwait)
set(USE_POSIX_SEM 1 CACHE BOOL "WASI threads libc result")
else()
set(USE_POSIX_SEM 0 CACHE BOOL "WASI libc result")
endif()
endif()


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35 changes: 29 additions & 6 deletions cmake/toolchains/WASI.cmake
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Expand Up @@ -9,16 +9,22 @@ set(CMAKE_SYSTEM_PROCESSOR wasm32)
set(SWIFT_WASI_TOOLCHAIN_PATH "${SWIFT_WASI_TOOLCHAIN_PATH}" CACHE PATH
"Host Swift .xctoolchain used to build for WASI")
set(SWIFT_WASI_SDK_PATH "${SWIFT_WASI_SDK_PATH}" CACHE PATH
"wasm32-unknown-wasip1 directory in a Swift WASI SDK")
"wasm32-unknown-wasip1 or wasm32-unknown-wasip1-threads directory in a Swift WASI SDK")
set(SWIFT_WASI_STATIC_RESOURCES_OVERRIDE "" CACHE PATH
"Optional override for the Swift static resource directory")
set(DISPATCH_WASI_BUILTINS_OVERRIDE "" CACHE FILEPATH
"Optional override for the WASI compiler-rt builtins archive")
option(DISPATCH_WASI_THREADS
"Experimental: target wasm32-unknown-wasip1-threads (wasi-threads hosts only)" OFF)
set(DISPATCH_WASI_MAX_MEMORY "268435456" CACHE STRING
"Shared-memory maximum in bytes for threads builds (wasm memories need a hard cap)")
list(APPEND CMAKE_TRY_COMPILE_PLATFORM_VARIABLES
SWIFT_WASI_TOOLCHAIN_PATH
SWIFT_WASI_SDK_PATH
SWIFT_WASI_STATIC_RESOURCES_OVERRIDE
DISPATCH_WASI_BUILTINS_OVERRIDE)
DISPATCH_WASI_BUILTINS_OVERRIDE
DISPATCH_WASI_THREADS
DISPATCH_WASI_MAX_MEMORY)

if(NOT SWIFT_WASI_TOOLCHAIN_PATH)
message(FATAL_ERROR "Set SWIFT_WASI_TOOLCHAIN_PATH to the host Swift .xctoolchain")
Expand Down Expand Up @@ -85,25 +91,42 @@ if(ENABLE_SWIFT)
endif()
endif()

if(DISPATCH_WASI_THREADS)
set(_dispatch_wasi_triple wasm32-unknown-wasip1-threads)
else()
set(_dispatch_wasi_triple wasm32-unknown-wasip1)
endif()

set(CMAKE_C_COMPILER "${_dispatch_wasi_clang}")
set(CMAKE_CXX_COMPILER "${_dispatch_wasi_clangxx}")
set(CMAKE_AR "${_dispatch_wasi_ar}")
set(CMAKE_RANLIB "${_dispatch_wasi_ranlib}")
set(CMAKE_C_COMPILER_TARGET wasm32-unknown-wasip1)
set(CMAKE_CXX_COMPILER_TARGET wasm32-unknown-wasip1)
set(CMAKE_C_COMPILER_TARGET ${_dispatch_wasi_triple})
set(CMAKE_CXX_COMPILER_TARGET ${_dispatch_wasi_triple})
set(CMAKE_SYSROOT "${_dispatch_wasi_sysroot}")
set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
set(CMAKE_EXECUTABLE_SUFFIX .wasm)

if(DISPATCH_WASI_THREADS)
# -pthread turns on atomics and shared memory. The memory must also be
# imported and exported with a hard maximum: wasi-threads hosts spawn
# each thread against the same imported shared memory, and a module
# that owns a private memory fails at the first pthread_create.
string(APPEND CMAKE_C_FLAGS_INIT " -pthread")
string(APPEND CMAKE_CXX_FLAGS_INIT " -pthread")
string(APPEND CMAKE_EXE_LINKER_FLAGS_INIT
" -Wl,--import-memory,--export-memory,--max-memory=${DISPATCH_WASI_MAX_MEMORY}")
endif()

if(ENABLE_SWIFT)
if(NOT EXISTS "${_dispatch_wasi_swiftc}")
message(FATAL_ERROR "Swift compiler does not exist: ${_dispatch_wasi_swiftc}")
endif()
set(CMAKE_Swift_COMPILER "${_dispatch_wasi_swiftc}")
set(CMAKE_Swift_COMPILER_TARGET wasm32-unknown-wasip1)
set(CMAKE_Swift_COMPILER_TARGET ${_dispatch_wasi_triple})
set(CMAKE_Swift_FLAGS
"-sdk \"${CMAKE_SYSROOT}\" -resource-dir \"${SWIFT_WASI_STATIC_RESOURCES}\"")
set(dispatch_MODULE_TRIPLE wasm32-unknown-wasip1 CACHE STRING "Swift module triple")
set(dispatch_MODULE_TRIPLE ${_dispatch_wasi_triple} CACHE STRING "Swift module triple")
set(dispatch_ARCH wasm32 CACHE STRING "Swift architecture")
set(dispatch_PLATFORM wasi CACHE STRING "Swift platform")
endif()
10 changes: 8 additions & 2 deletions src/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -39,6 +39,7 @@ add_library(dispatch
event/event_internal.h
event/event_kevent.c
event/event_wasi.c
event/event_wasi_threads.c
event/event_windows.c
firehose/firehose_internal.h
shims/android_stubs.h
Expand Down Expand Up @@ -107,8 +108,13 @@ elseif(ANDROID)
target_compile_options(dispatch PRIVATE
-U_GNU_SOURCE)
elseif(CMAKE_SYSTEM_NAME STREQUAL "WASI")
target_compile_definitions(dispatch PRIVATE
DISPATCH_HW_CONFIG_UP=1)
if(NOT DISPATCH_WASI_THREADS)
# single-threaded wasip1 is a uniprocessor by construction; the threads
# target is not (and DISPATCH_HW_CONFIG_UP also shrinks continuations
# below sizeof(struct dispatch_apply_s) there)
target_compile_definitions(dispatch PRIVATE
DISPATCH_HW_CONFIG_UP=1)
endif()
target_compile_definitions(dispatch PUBLIC
"$<$<COMPILE_LANGUAGE:C,CXX>:_WASI_EMULATED_SIGNAL>"
"$<$<COMPILE_LANGUAGE:C,CXX>:_WASI_EMULATED_MMAN>"
Expand Down
14 changes: 13 additions & 1 deletion src/event/event_config.h
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Original file line number Diff line number Diff line change
Expand Up @@ -21,27 +21,39 @@
#ifndef __DISPATCH_EVENT_EVENT_CONFIG__
#define __DISPATCH_EVENT_EVENT_CONFIG__

#if defined(__wasi__)
#if defined(__wasi__) && !defined(_REENTRANT)
# define DISPATCH_EVENT_BACKEND_EPOLL 0
# define DISPATCH_EVENT_BACKEND_KEVENT 0
# define DISPATCH_EVENT_BACKEND_WASI 1
# define DISPATCH_EVENT_BACKEND_WASI_THREADS 0
# define DISPATCH_EVENT_BACKEND_WINDOWS 0
#elif defined(__wasi__)
// wasm32-wasip1-threads: experimental threaded mode; a manager thread on a
// condition variable replaces the cooperative drain (event_wasi_threads.c)
# define DISPATCH_EVENT_BACKEND_EPOLL 0
# define DISPATCH_EVENT_BACKEND_KEVENT 0
# define DISPATCH_EVENT_BACKEND_WASI 0
# define DISPATCH_EVENT_BACKEND_WASI_THREADS 1
# define DISPATCH_EVENT_BACKEND_WINDOWS 0
#elif defined(__linux__)
# include <sys/eventfd.h>
# define DISPATCH_EVENT_BACKEND_EPOLL 1
# define DISPATCH_EVENT_BACKEND_KEVENT 0
# define DISPATCH_EVENT_BACKEND_WASI 0
# define DISPATCH_EVENT_BACKEND_WASI_THREADS 0
# define DISPATCH_EVENT_BACKEND_WINDOWS 0
#elif __has_include(<sys/event.h>)
# include <sys/event.h>
# define DISPATCH_EVENT_BACKEND_EPOLL 0
# define DISPATCH_EVENT_BACKEND_KEVENT 1
# define DISPATCH_EVENT_BACKEND_WASI 0
# define DISPATCH_EVENT_BACKEND_WASI_THREADS 0
# define DISPATCH_EVENT_BACKEND_WINDOWS 0
#elif defined(_WIN32)
# define DISPATCH_EVENT_BACKEND_EPOLL 0
# define DISPATCH_EVENT_BACKEND_KEVENT 0
# define DISPATCH_EVENT_BACKEND_WASI 0
# define DISPATCH_EVENT_BACKEND_WASI_THREADS 0
# define DISPATCH_EVENT_BACKEND_WINDOWS 1
#else
# error unsupported event loop
Expand Down
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